An interesting comparison (not mine)

I have Canon's 14mm L II and have used it quite a bit for UWA on a 1DM4 but, on a FF camera, it suffers greatly from CA and soft corners (even at f11 in the foreground of landscape shots). Then I bought the 17mm TSE. IMHO, on FF, I don't think there is a better IQ lens made for Canon under 20mm. Let's face it, on FF you rarely need wider than 17mm and given that is not much more than the Canon 14mm L, I'd opt for it ever time if looking in that price range. The bonus of course is the TSE fuction...